1. Field of the Invention
The present invention relates to methods and apparatus for making electrical measurements in the presence of periodic electromagnetic fields and more particularly to such methods and apparatus which enable electrical measurement of a low amplitude signal of interest in the presence of a high energy oscillator.
2. Description of the Related Art
Accurate measurement of low amplitude signals of interest in the presence of an oscillating electromagnetic field poses a problem. The oscillating field imposes periodic noise on the conductor(s) carrying the signal thus producing inaccurate measurements. This is especially so in the case where a signal of interest is periodically sampled, held and then converted to a digital value because sampling may occur at different phases of the periodic noise signal. Even if the sampling rate is synchronized with the periodic noise signal, variations in the length of time for the digital conversion to occur permit random noise levels to appear on the held signal.
One instrument in which such problems arise is a cardiograph. A cardiograph includes a plurality of leadwires which are connected to a patient's skin via electrodes for sensing electrical signals generated by the patient's heart. Such signals are low amplitude and have a bandwidth of roughly 0.5 to 150 hertz. A cardiograph includes one or more power supplies which each incorporate a transformer or inductor to which an oscillating voltage signal is applied. The power supplies generate periodic electromagnetic fields which are induced on the leadwires and other conductors carrying the signals of interest from the patient.
In one prior art cardiograph, an analog signal of interest is sampled, and thereafter held, at a rate which is an integer multiple of the power supply frequency. In the prior art conversion process, the signal is sampled and held in an isolated portion of the cardiograph. The held signal is pulse width modulated, i.e., a pulse having a width proportional to the magnitude of the held signal is generated. A synchronizing signal is taken from the secondary of an isolation transformer which provides power to the isolated circuit. Responsive to the synchronizing signal, a digital word representing the pulse width is generated and transmitted via an isolation coupling to a nonisolated portion of the cardiograph.
This prior art system suffers from a disadvantage in that the time during which the analog signal is converted depends upon the magnitude of the held signal. Because the duration of the conversion process is variable, and not necessarily in phase with the periodic noise, inaccuracies are injected into the conversion process.
In another prior art analog-to-digital conversion process implemented in a cardiograph, the dc offset is subtracted from an analog signal and the resulting signal is amplified. The amplified signal is sampled and held with the held signal being provided to an analog-to-digital converter of the dual-slope integration type. The signals which initiate each conversion process are synchronized to the power supply. Each conversion process is thus initiated at the same phase of the power supply signal as is the case with the pulse-width-modulated system described above. This prior art system is complex in that preconditioning of the analog signal, including subtracting the dc offset, is required.
In an effort to minimize the effects of periodic noise induced by power supplies, prior art cardiographs utilize expensive and bulky shielding around power supplies to limit radiation of the oscillating field. Prior art cardiographs also use filtering schemes to minimize noise in the signals of interest. Such filtering may reduce noise but it has the disadvantage of modifying information contained in the signals of interest.